The
Mineral CACOXENITE

Cacoxenite is a mineral that is commonly an inclusion in quartz,
especially amethyst.
The inclusions in amethyst often detract from the amethyst as the brown
acicular needles dampen the pure purple color to a less desirable brownish
hue. The inclusions of cacoxenite will certainly ruin any chance for the
host stone to become a gemstone. This is not to say that some specimens
of cacoxenite included quartz have no value, for indeed some are actually
enhanced with interesting surreal landscaping effects.

Cacoxenite on its own is appreciated as a scarce phosphate mineral and
is known from classic phosphate localities. It is often associated with
other attractive and rare phosphates and can therefore represent some very
nice mineral specimens. These specimens can be quite popular and attractive
with a silky luster and a typical yellow-brown color.

The rather daunting formula of cacoxenite, Fe24(AlO6)(PO4)17(OH)12
- 75H2O, can be represented in a more condensed
and simpler version, Fe4(PO4)3(OH)3
- 12H2O. This shorter version is what is often
used in some references. The shorter version is approximately one sixth
of the larger one except for the addition of one aluminum and an adjustment
to the number of oxygens and water molecules. This can be demonstrated:

Fe24 (AlO6)
(PO4)17 (OH)12
- 75H2O

The aluminum group occupies a phosphate position in the structure
and this can be shown as follows:

Fe24 [(AlO6),
(PO4)]18 (OH)12
- 75H2O

Combine the 6 oxygens to 6 hydrogens from three water molecules
to form 6 more OH's:

Fe24 [Al, (PO4)]18
(OH)18 - 72H2O

Then the formula can be shown as:

Fe(6x4) [Al, (PO4)](6x3)
(OH)(6x3) - 6x12H2O
;

factoring out 6 and ignoring the aluminum gives:

6 x (Fe4 (PO4)3
(OH)3 - 12H2O)

This change requires the loss of three oxygens in order to balance the
formula due to the change in charge from one negative 3 phosphate group
to the negative 9 aluminum group. Natural cacoxenite contains one aluminum
group for every 17 phosphates making the larger formula more accurate,
if not less cumbersome.